1. Field of the Invention
The present invention relates to a developing apparatus that can be used for an image forming apparatus, such as a copying machine, a facsimile machine, a printer and a combined copying machine. The developing apparatus can be provided in a cartridge detachably mountable to the main body of the image forming apparatus.
2. Description of the Related Art
Conventionally, a developing system using dual-component developer has been frequently used in general, but in recent years, there has been a growth in the use of developing apparatuses using mono-component developer that are simple in construction and advantageous also in running cost. Also, there is available a replenishing system for replenishing a developer (hereinafter simply referred to as “toner”) in order to maintain image quality satisfactory until it reaches the end of developer life. Even in this case, however, the use of the mono-component developer is being substituted for the dual-component developer. In addition, a developing apparatus with the aim of downsizing by adopting vertical circulation of toner is also proposed (refer to, for example, Japanese Patent Application Laid-Open No. H11-024382).
In the replenishing system of a developer, there are differences in physical properties such as charge amount and flowability between an existing toner that currently exists in a developing chamber and a new replenished toner that is replenished, and the existing toner and the replenished toner having different physical properties from each other coexist in one developing chamber. In this case, when physical properties of both the existing toner and the replenished toner are greatly different, so-called coarseness or fog on a white background portion may occur in a half-tone image, and there occur defects such as a dropping on sheet. To resolve such defects, there are many developing apparatuses with a construction to convey a toner while agitating adequately the replenished toner with the existing toner.
FIG. 9 and FIG. 10 illustrate one example of a developing apparatus of such a replenishing system and a toner circulation configuration. A developing apparatus 11 is comprised of two chambers, an agitating chamber 9 and a developing chamber 10. The agitating chamber 9 and the developing chamber 10 are separated from each other by a partition wall 16, and an inlet 19 and an outlet 20 are provided at both ends of the partition wall 16 and opened in the partition wall 16 so as to allow the toner to flow to and from. In the developing chamber 10, a developing roller 1, a toner supply roller 2, a toner regulating member 3 and a developing container 12 are accommodated therein. Also, a screw 4 is arranged in parallel with the toner supply roller 2, and the agitating chamber 9 is arranged at the opposite side of the screw 4 sandwiching the partition wall 16. The screw 4 is provided at a higher level in the gravitational direction than an axis of a rotation center of the toner supply roller 2, in order to carry out two functions of supplying the toner to the toner supply roller 2 and returning the toner to the agitating chamber 9.
A toner replenishing operation corresponding to the detection of the toner amount in the agitating chamber 9 as well as the obtained information regarding coverage rates of images is performed by a replenishing device 8, then the replenished toner is dropped off into the agitating chamber 9 after passing through an opening 6. An agitating member 5 arranged in the agitating chamber 9 can level the toner horizontally, but does not have further conveying capability. Accordingly, even if the agitating member 5 in the agitating chamber 9 attempts to rotate to the limit, there are no cases where it actively feeds the toner to the inlet 19. The movement of the toner in the agitating chamber 9 is such that the toner conveyed from the side of inlet 19 and the replenished toner will rise high at the outlet 20 within the agitating chamber 9. The agitating member 5 rotates repeatedly to level the toner horizontally, and then spreads it gradually throughout the agitating chamber 9, as indicated by arrows D. By repeating this operation the toner finally reaches the inlet 19 to pass through the opening, and drops into the developing chamber 10 with the aid of the gravitational force of the toner so as to be supplied to the toner supply roller 2.
The developing roller 1 and the toner supply roller 2 rotate in the directions as indicated by the arrows in FIG. 9. The developing roller 1 is coated with a toner due to rotational friction, and the toner is scraped to be leveled when it passes through a toner regulating member 3 abutting on the developing roller 1 as the developing roller 1 rotates, so that a thinned toner coat layer is formed on the developing roller 1. The toner that has not been expended by reason of low coverage rate is scraped off from the developing roller 1 by the toner supply roller 2, and subsequently conveyed by the screw 4, passing through the outlet 20 so as to be returned to the agitating chamber 9. Thus, a toner circulation in vertical direction is created, and the replenished toner and the existing toner are adequately agitated and conveyed.
Incidentally, in a developing apparatus of a replenishing system, a horizontal in-line system is commonly used, whereas a construction to circulate the toner in the gravitational direction, i.e. an up-and-down vertical direction would offer such an advantage that a width size of the entire developing apparatus can be reduced to achieve downsizing, which is very effective.
As illustrated in FIG. 10, the toner in the developing chamber 10 is conveyed by the screw 4 in a direction from the one side to the other side of the longitudinal direction, and fed by its conveying pressure C from the opening of the outlet 20 to the agitating chamber 9. Also, a toner replenished from the toner replenishing mechanism 8 is replenished to the side of the outlet 20 in the agitating chamber 9 in a similar way, since it takes an agitating time in the agitating chamber 9. The replenished toner is agitated and the surface 18 is leveled horizontally by the agitating member 5. The toner replenished by repeating this movement finally reaches the inlet 19 by the movement of spreading the toner over the agitating chamber 9 as indicated by the arrows D, and supplied from the inlet 19 into the developing chamber 10, as shown by arrow A.
On the other hand, when images with a high coverage rate are continuously printed, a difference in an image density likely occurs in the longitudinal direction of the developing apparatus, that is, in the toner conveying direction in which the toner is conveyed by the screw 4 of the developing apparatus in the direction indicated by arrow AC in FIG. 10, and as shown in a characteristic graph of FIG. 11 and in a density measuring point diagram of FIG. 12. This is because the presence ratio of the replenished toner in the agitating chamber 9 becomes more than that of the existing toner due to the continuous toner replenishing operation, as a result, the presence ratio of the replenished toner becomes high in the toner layer on the developing roller 1 in the vicinity of the inlet 19 of the developing chamber 10. In other words, in a circulation route having the inlet 19 and the outlet 20, the toner supplied from the inlet 19 is much coated on the developing roller 1 on the upstream side in the toner conveying direction.
Since the replenished toner has a high charge amount, the coat tends to be thin on the developing roller 1. On the contrary, since the existing toner has a low charge amount, the coat on the developing roller 1 tends to be thick. As a result, when a high portion and a low portion of the presence ratios of the replenished toner appear on the developing roller 1, the density becomes light at a high portion of the presence ratio of the replenished toner, whereas the density becomes dense at a low portion of the presence ratio of the replenished toner. This phenomenon noticeably occurs particularly under a high-temperature and high-humidity environment where difference in charge amount is likely to occur between the existing toner and the replenished toner.
Here, as illustrated in FIG. 13, if a cross-sectional area of a region of the developing chamber 10 surrounded by the developing container 12, excluding a conveying region of the screw 4, in a higher level region in the gravitational direction than the rotational axis of the supply roller 2 is expressed as S, the cross-sectional area S of the developing chamber 10 is constant at each point. It should be noted that the conveying region of the screw 4 is meant by a region covered by the uppermost point and the lowermost point of the screw shape.
Incidentally, there are common problems to be solved in the developing apparatus disclosed in Japanese Patent Application Laid-Open No. H11-024382 and the developing apparatus of the replenishing system as shown in FIG. 13.
Since the cross-sectional area S of the developing chamber 10 is constant at each point in the longitudinal direction, a force in a flow direction E caused by a toner blowoff is equal at each point in the longitudinal direction. Therefore, as illustrated schematically in FIG. 14, the flow of toner supplied from the inlet 19 as indicated by the arrows F is large on the upstream side in toner conveying direction, whereas the flow F of the toner supplied from the inlet 19 becomes gradually smaller toward the downstream side Even from this fact, the toner density has an impact on an unstable shading.
As described above, in whichever case where a toner is replenished under the impact of a cross-sectional construction of the developing chamber 10 and a high-temperature and high-humidity environment, and a print setting varies each time, it is strongly desired that a toner be coated uniformly on the developing roller 1, and the densities on the upstream side and downstream side in the toner conveying direction be uniformly maintained.